You can dispense with the need for an input scalar by opening a filehandle on a reference to a HEREDOC. Also, there's no need to initialise your output scalar to the empty string as any previous content will be clobbered by the '>' in the open.

knoppix@Microknoppix:~$ perl -e '
> open my $inFH, q{<}, \ <<EOD or die qq{open: << HEREDOC: > $!\n};
> line 1
> line 2
> line 3
> EOD
> 
> my $out = qq{some rubbish here\n};
> open $outFH, q{>}, \ $out or die qq{open: > scalar: $!\n};
> 
> while ( <$inFH> )
> {
>     print $outFH uc;
> }
> 
> print $out;'
LINE 1
LINE 2
LINE 3
knoppix@Microknoppix:~$
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I hope this is of interest.