#  Rejected possibility one:
substr( $string, 0, length( $string ) / 2 ) =~ m/pattern/;

#  Rejected possibility two:
my $half = length( $string ) / 2;
$string =~ m/pattern.{$half}/s;

##</code><code>##

use strict;         # Because you should.
use warnings;       # Because you want to.
use re 'eval';      # Take your life in your own hands.

my $find = qr/123/;   # This is what we're searching for.

while ( <DATA> ) {
    chomp;
    if (
          m/$find                        # Match the substring.
            (?{
               (pos()<=length($_)*.5)    # Test to see if our match
                                         # occurred in the first half
                                         # of the string.
               ? ''                      # Yes: pass a subexpression
                                         # that can't fail.
               : '\w\b\w'                # No: pass a subexpression
                                         # that always fails.
            })
            (??{$^R})                    # Evaluate the passed
                                         # subexpression.
          /x
    ) {
        print "$_ matched.\n";
    } else {
        print "$_ didn't match.\n";
    }
}


__DATA__
1230000000
0123000000
0012300000
0001230000
0000123000
0000012300
0000001230
0000000123

##</code><code>##

1230000000 matched.
0123000000 matched.
0012300000 matched.
0001230000 didn't match.
0000123000 didn't match.
0000012300 didn't match.
0000001230 didn't match.
0000000123 didn't match.

##</code><code>##

use strict;
use warnings;
use re 'eval';

my $find = qr/123/;

while ( <DATA> ) {
    chomp;
    if ( m/$find(?(?{(pos()<=length($_)*.5)})|\w\b\w)/ ) {
        print "$_ matched.\n";
    } else {
        print "$_ didn't match.\n";
    }
}


__DATA__
1230000000
0123000000
0012300000
0001230000
0000123000
0000012300
0000001230
0000000123