You may be getting into swap by reading the whole file into an array. A while loop on the filehandle will read one line at a time into $_, keeping memory requirements to a minimum.

open my $fh, '<', '/path/to/file'
    or die $!;
while (<$fh>) {
    if ( $_ eq $external_info ) {
        print 'great!', $/;
        last;
    }
}
close $fh or die $!;
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You will need to chomp at the top of the while loop if $external_info does not have a newline at the end.

I converted to 3-arg open, and used a lexical file handle for that. You can convert back to suit previous versions of perl.

After Compline,
Zaxo