use strict;
use warnings;
use File::Temp;
use File::Copy qw(move);
use Fcntl qw(:flock);

my $target_file = 'generic_bist_ctrl_defines.sv';

{
    open my $in_fh, '<', $target_file or die "Cannot open input file: $!\n";
    flock $in_fh, LOCK_EX | LOCK_NB or die "Cannot obtain a lock on $target_file: $!\n";

    my $temp_out = File::Temp->new(TEMPLATE => "$0-$$-XXXXX");

    while (my $line = <$in_fh>) {
        $line =~ s/=131/= $a/g; # Did you actually mean $line =~ s/=(131)/= $1/g; ???
        print $temp_out $line;
    }
    close $in_fh or die "Failed to close $target_file. Aborting. $!\n";

    $temp_out->flush;
    eval {
        move($temp_out->filename, $target_file);
    } or do {
        warn "Failed to swap $temp_out into $target_file: $!\n";
        # There may need to be more cleanup here, or possibly a die is more appropriate.
    };
}   # $temp_out falls out of scope; temp file should be removed even if the move call failed.
    # flock falls out of scope; lock is released on $target_file.