How about approaching from a slightly different angle: grab the bits of info you do want from the string, then use them to construct a new string.

Do a regex match on your initial string, using parens and array context to get back the pieces you're interested in, like so:

($month, $day, $hour, $min, $ampm) = ($stamp =~ m|(\d+)/(\d+)/\d+\s+(\
+d+):(\d+)(\w)|);
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Then use those pieces to build your new string:

$newstamp = sprintf("%02d_%02d_%02d%02d%s", $month, $day, $hour, $min,
+ $ampm);
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(Using %02d in sprintf will give you leading zeros in front of single digit numbers, which will make your resulting string more consistent -- change them to just %d to leave off the leading zeros)

This has the added benefit of making the code's function much more obvious 6 months from now than a cryptic series of substitutions, and being easier to update if and when your formats change.