I would probably re-write it a bit

untested

use strict;
use warnings;

open (IN, 'myfile.txt') or die $!;
my @lines = <IN>;
my $cnt = 0;

for my $line (@lines) {
    if ($line =~ /somekeyword/) {
        $cnt++;
    }
}
print ("somekeyword was found in $cnt lines\n");
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You could shorten this code a LOT but wanted to make it a little more explicit.

NOTE: You don't have to necessarily put the whole file in an array. You can use while(<IN> and check that way too.