use strict;
use warnings;
use autodie;

my $data = <<'EOD';

input  bist_g6bb_prn_clk_m;
EOD

open my $fh, '<', \$data;

while (my $line = <$fh>) {
    $line =~ s/input .*clk/output .*clk/g;
    print $line;
    }

close $fh;