Or if you're on a windows machine without grep perl -ne'print if /TWO/' b2bclient-gateway-heartbeat.log > output.txt

If it is part of a larger task then

my $in_file = 'b2bclient-gateway-heartbeat.log';
open my $in_fh, '<', $in_file or die "Could not open file $in_file: $!
+";

my $out_file = 'output.txt';
open my $out_fh, '>', $out_file or die "Could not open file $out_file:
+ $!";

while ( my $line = <$in_fh> ) {
    print {$out_fh} $line if $line =~ /TWO/;
}

close $in_fh  or die "Could not close file $in_file: $!";
close $out_fh or die "Could not close file $out_file: $!";
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