If you're using variable interpolation, Perl may not always understand exactly what you intend. To get your code to print "yesgoodbye", try putting braces around the variable to force Perl to recognize exactly what variable it's interpolating:

print "${hello}goodbye";
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Update: I find it somewhat amusing that no one (as of this posting) pointed fx's obvious solution. So I can't help but wonder, are there inherent problems with his solution, or does one person suggesting a solution lead the PerlSheep (that would include me) down a narrow path?