If you're going to base something on chomp, you should use the same interface. In other words, you should work in-place.

sub chimpx {
    my $pat = @_ ? shift : $/; for @_ ? @_ : $_;
}
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If you want do a non-destructive chimp, use

chimpx $pat, my $dst = $src;
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Update: ok, since I didn't preserve chomp's return value, let's do it your way:

sub chimpx {
    my $pat = @_ ? shift : $/;
    my $targs = sub { \@_ }->( @_ ? @_ : $_ );
    @$targs = @$targs if defined(wantarray);
    s/^(?:\Q$pat\E)*// for @$targs;
    return wantarray ? @$targs : $targs->[0];
}
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Tests:

use Test::More tests => 18;

{
    local $_ = 'ababac';
    my $x = 'ababac';
    chimpx 'ab', $x;
    is($_, 'ababac', 'void pat var - $_ preservation');
    is($x, 'ac',     'void pat var - result verification');
}

{
    local $_ = 'ababac';
    my $x = 'ababac';
    my $y = chimpx 'ab', $x;
    is($_, 'ababac', 'scalar pat var - $_ preservation');
    is($x, 'ababac', 'scalar pat var - arg preservation');
    is($y, 'ac',     'scalar pat var - result verification');
}

{
    local $_ = 'ababac';
    chimpx 'ab';
    is($_, 'ac', 'void pat novar - result verification');
}

{
    local $_ = 'ababac';
    my $y = chimpx 'ab';
    is($_, 'ababac', 'scalar pat novar - $_ preservation');
    is($y, 'ac',     'scalar pat novar - result verification');
}

{
    local $_ = "$/$/c";
    chimpx;
    is($_, 'c',      'void nopat novar - result verification');
}

{
    local $_ = "$/$/c";
    my $y = chimpx;
    is($_, "$/$/c", 'scalar nopat novar - $_ preservation');
    is($y, 'c',     'scalar nopat novar - result verification');
}

{
    local $/ = 'ab';
    local $_ = "$/$/ac";
    chimpx;
    is($_, 'ac', 'alt $/ - result verification');
}

{
    my $x1 = 'ababac';
    my $x2 = 'ababad';
    chimpx('ab', $x1, $x2);
    is($x1, 'ac', 'void multiple args - 1st result verification');
    is($x2, 'ad', 'void multiple args - 2nd result verification');
}

{
    my $x1 = 'ababac';
    my $x2 = 'ababad';
    my ($y1, $y2) = chimpx('ab', $x1, $x2);
    is($x1, 'ababac', 'scalar multi args - 1st arg preservation');
    is($x2, 'ababad', 'scalar multi args - 2nd arg preservation');
    is($y1, 'ac',     'scalar multi args - 1st result verification');
    is($y2, 'ad',     'scalar multi args - 2nd result verification');
}
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