Default Arguments

A subroutine can fill in a missing argument with a default value.
default argument A default argument supplies a fallback when the caller omits a value.
                            count
                            count = 1 (default)count = 0count = 3
                        

                            default_arguments.pl
                        
Replay: real traced execution (multi-file project)

                            use strict;
use warnings;

sub count_label {
    my ($count, $noun) = @_;
    $noun //= "item";
    my $label = $count == 1 ? $noun : $noun . "s";
    return "$count $label";
}

my $count = 1;
my $text = count_label($count);

print "count=$count\n";
print "text=$text\n";
use strict;
use warnings;

sub count_label {
    my ($count, $noun) = @_;
    $noun //= "item";
    my $label = $count == 1 ? $noun : $noun . "s";
    return "$count $label";
}

my $count = 0;
my $text = count_label($count);

print "count=$count\n";
print "text=$text\n";
use strict;
use warnings;

sub count_label {
    my ($count, $noun) = @_;
    $noun //= "item";
    my $label = $count == 1 ? $noun : $noun . "s";
    return "$count $label";
}

my $count = 3;
my $text = count_label($count);

print "count=$count\n";
print "text=$text\n";

                        
1$count ← 1⋯11my $count→ 1 = 1; #@count=0, 312my $text = count_label($count1);⋯
2$count ← 1, $noun ← (empty), $label ← item⋯4sub count_label {5    my ($count→ 1, $noun→ (empty)) = @_;6    $noun→ item //= "item";7    my $label→ item = $count1 == 1 ? $noun→ item : $noun . "s";8    return "$count1 $labelitem";9}⋯
3$text ← 1 item⋯11my $count = 1; #@count=0, 312my $text→ 1 item = count_label($count1);1314print "count=$count1\n";15print "text=$text1 item\n";⋯outputcount=1
text=1 item
1$count ← 0⋯11my $count→ 0 = 0;12my $text = count_label($count0);⋯
2$count ← 0, $noun ← (empty), $label ← items⋯4sub count_label {5    my ($count→ 0, $noun→ (empty)) = @_;6    $noun→ item //= "item";7    my $label→ items = $count0 == 1 ? $noun→ item : $noun . "s";8    return "$count0 $labelitems";9}⋯
3$text ← 0 items⋯11my $count = 0;12my $text→ 0 items = count_label($count0);1314print "count=$count0\n";15print "text=$text0 items\n";⋯outputcount=0
text=0 items
1$count ← 3⋯11my $count→ 3 = 3;12my $text = count_label($count3);⋯
2$count ← 3, $noun ← (empty), $label ← items⋯4sub count_label {5    my ($count→ 3, $noun→ (empty)) = @_;6    $noun→ item //= "item";7    my $label→ items = $count3 == 1 ? $noun→ item : $noun . "s";8    return "$count3 $labelitems";9}⋯
3$text ← 3 items⋯11my $count = 3;12my $text→ 3 items = count_label($count3);1314print "count=$count3\n";15print "text=$text3 items\n";⋯outputcount=3
text=3 items