Minor nitpick for efficiency: don't do join '', <FH> - ever. Since you're only going to glue all the lines together anyway, don't ask for the file to be delivered line by line in the first place. Either localize $/ or use read. I'd do the first in this case.

if(..) {
    open my $fh, "<", $filename or die &cant_open_file;
    local $/ = \8192; # read in 8 kbyte chunks rather than line by lin
+e
    print while <$fh>;
}
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See perldoc perlvar on the intricacies of $/. Note that this code uses a lexical to store the filehandle - it needs Perl 5.6 or newer to work that way, but affords us the luxury of not closing the file explicity. It just gets autoclosed when the $fh goes out of scope at the end of the block.

Makeshifts last the longest.