Is it possible to simplify my procedure? Could it be more perlish?

1) Get rid of garbage like $_[0], $_1. Instead use shift():

sub do_stuff {
    my $dir = shift;
    my $pattern = shift;
    ...
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... or list assignment if you want to keep @_ intact:

my ($dir, $pattern) = @_;
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Do either of those as the *first* line(s) of your sub.

2) In general, your variable names are atrocious:

$what     --> $pattern_str
@t        --> @patterns
$f        --> $pattern 
$file2md5 --> $fname
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3) As an alternative to split() and looping, you could chdir($dir) and then glob($pattern_str).

4) What do you think the quote marks do here:

"$file2md5"
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5) Don't use bareword filehandles. Use a my() variable instead(which you can capitalize if you want to):

my $fname_out = 'name.txt';

open my $OUTFILE, '>', $fname_out
  or die "Couldn't open $fname_out: $!";

#and then...
print $OUTFILE "whatever";
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