Hi try this,

use strict;
my $toFind="String";

while(<DATA>){
    my $line=$_;
    chomp($line);    
    if($line=~m/\b$toFind(?=[^\w\.]+|$)/i){
        print "$line\n";
    }
}
[download]

__DATA__
blah blah string blah
important stringstuff we wish to keep
Unimportant string
more stuff we wish to String.o
Some more unimportant String.c
[download]

Punitha