## Either
substr( $x, 0, index( $x, $char ), '');
## or
$x =~ s[(^/*$char)][];

##</code><code>##

for my $char ( unpack '(A1)*', substr $x, 0, 100, '' ) {
    ##
}

##</code><code>##

use strict;
use warnings;

use Benchmark qw( cmpthese );

my %tests = (
   subst       => '$x =~ s/.//;',
   substr_lval => 'substr($x,0,1) = "";',
   substr_mod  => 'substr($x,0,1,"");',
   reverse     => '$x = reverse $x; chop($x); $x = reverse($x);',
   substr_copy => '$x = substr($x,1);',
);

our $loops ||= 1e4;
for (values %tests) {
    $_ = <<EOT
    my \$x = 'X' x $loops;
    for( 1 .. $loops ) {
       $_
    }
EOT

}

cmpthese(-3, \%tests);