sub foo {
    ...
    my $var = "<value>"
        if <condition>;
    ...
}

##</code><code>##

$ cat my_with_if.pl
#!/usr/bin/env perl

use strict;
use warnings;
use feature 'say';
use feature 'signatures';
no warnings 'experimental::signatures';

use Data::Dump qw( pp );

sub foo( $value ) {
    say "foo( ", pp( $value ), " )";

    my $result = "default"
        if not defined $value;

    say "\$result after 'my' is ", pp( $result );
    $result //= $value;
    say "\$result after '//=' is ", pp( $result );
    say "";
    return $result;
}

foo( $_ )
    for (
        "call 1.1",
        "call 1.2",
        "call 1.3",
        undef,
        "call 2.1",
        "call 2.2",
        "call 2.3",
    );

1;

$ ./my_with_if.pl
foo( "call 1.1" )
$result after 'my' is undef
$result after '//=' is "call 1.1"

foo( "call 1.2" )
$result after 'my' is "call 1.1"
$result after '//=' is "call 1.1"

foo( "call 1.3" )
$result after 'my' is "call 1.1"
$result after '//=' is "call 1.1"

foo( undef )
$result after 'my' is "default"
$result after '//=' is "default"

foo( "call 2.1" )
$result after 'my' is undef
$result after '//=' is "call 2.1"

foo( "call 2.2" )
$result after 'my' is "call 2.1"
$result after '//=' is "call 2.1"

foo( "call 2.3" )
$result after 'my' is "call 2.1"
$result after '//=' is "call 2.1"


##</code><code>##

 
    my $var = <condition> ? "<value>" : undef;