Is there a way to do this without using a non-core module?

Update: Added required ->'s per choroba's offline observation. (Many thanks!)

You could write your own:

sub firstidx(&@) {
    my $code = shift;
    $code->( $_[ $_ ] ) and return $_ for 0 .. $#_;
}
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But it won't be as fast as the (XS) module.

A possibly better version::

sub firstidx(&@) {
    my $code = shift;
    $code->( local $_ = $_[ $_ ] ) and return $_ for 0 .. $#_;
}
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The most obvious way would be a loop.

$i = 0;
until( $F[$i] =~ m/MD:Z/ || $i > $#F )
  $i += 1
}
if ($i <= $#F) {
  print "It's field $i\n"
}
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