I'm not sure that there is a 'p' modifier for regular expressions (did you mean to use the '-p' command-line switch?) and there is no point in using the 'x' modifier if you don't use extended syntax. This can be done in a one-liner like this.

$ mkdir lloderl74
$ cat > lloderl74/script1
#!/usr/bin/perl -w
print "Hello World!\n";
exit;
$ cat > lloderl74/script2
#!/usr/bin/perl -w
die "Goodbye, Cruel World\n";
$ perl -pi.bak -e '
s{^(#!/usr/bin/perl.*)}{$1\n\nuse English q{no_match_vars}}' lloderl74
+/*
$ head -99 lloderl74/*
==> lloderl74/script1 <==
#!/usr/bin/perl -w

use English q{no_match_vars}
print "Hello World!\n";
exit;

==> lloderl74/script1.bak <==
#!/usr/bin/perl -w
print "Hello World!\n";
exit;

==> lloderl74/script2 <==
#!/usr/bin/perl -w

use English q{no_match_vars}
die "Goodbye, Cruel World\n";

==> lloderl74/script2.bak <==
#!/usr/bin/perl -w
die "Goodbye, Cruel World\n";
[download]

I hope this helps you.