Another way would be to match the portion before it. For example: 
my $point = 15;
if ($str =~ /^(.{15})/ && $1 =~ /(regex)$/) {
    print "stuff before pos 15: $1\n";
}

[download]
This way, you at least don't have to copy the full string before searching. You only copy as much as length($1).

If you know the approximate length of the string that will be matched, you can minimize what's copied even further by using substr(): 
my $point = 15;
my $start = 10;
if (substr($str, $start, $point - $start) =~ /(regex)$/) {
    print "stuff before pos: $1\n";
}

[download]
If you only need to know whether or not something matched, and not actually fetch the resulting match, you can use one of Perl 5.005's look-behind assertions: 
pos $str = 15;
print "matched\n" if $str =~ /\G(?<=regex)/g;

[download]
There are probably better ways to deal with this, but if you post an example string, a heuristic might arise that eliminates the need to do any of this.

In reply to Re: Backwards searching with regexps by jerji
in thread Backwards searching with regexps by Anonymous Monk

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