I have written the following demo program but was delayed for almost an hour because I had to attend the Monday morning's manager's meeting. Anyway, I will post it here anyway, since no monks have shown you a search and replace example yet. :-)

use strict;

my $string = '+5^7';
my $line = '((1+2)-4)*(2+5^7)';

# insert \ in front of pattern
$line =~ s/(\Q$string\E)/\\$1/;  

print "$line\n";
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Basically you use \Q and \E to quote meta characters, or use the 'quotemeta' function.

And the output is -

((1+2)-4)*(2\+5^7)
[download]