You want transparent? How about just doing this from a (Unix) command line (after removing all your debugging code):

$ PERLDB_OPTS="f=3 N LineInfo=xxx" perl -d imlate.pl
Default die handler restored.
hello!
goodbye!
hello!
goodbye!
i'm late!
i'm late!
i'm late!
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This will result in the file xxx containing your trace:

$ cat xxx
Package imlate.pl.
entering main::hello
 entering main::goodbye
 exited main::goodbye
 entering main::goodbye
 exited main::goodbye
exited main::hello
entering main::imlate
exited main::imlate
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By changing the settings of the "f=n" option, you can change the printout (from perldoc perldebguts):

In all cases shown above, the line indentation shows the call tree. If bit 2 of "frame" is set, a line is printed on exit from a subroutine as well. If bit 4 is set, the arguments are printed along with the caller info. If bit 8 is set, the arguments are printed even if they are tied or references. If bit 16 is set, the return value is printed, too.

Without the LineInfo=xxx, the entering/exited messages will end up on stdout (like in yours).