use strict;   # this will help you catch a LOT of errors

my $tail = "some string";

open(INF,"data.txt") or die "Couldn't open data file: $!";
my $linenum=0;
while (<INF>) {
    print "$linenum\n";
    if ($_ =~ /|$tail/) {
        # do stuff
    }
    $linenum++;
}
close(INF);