The idiom to execute perl statements on and print the result of each line of a file is
perl -pe 'perl_statements' < file
or
some_command | perl -pe 'perl_statements'

each line is stored in $_ on which perl_statements act.

Here you want to print all the files in the $HOME/Mail directory. An all perl solution is:
   map{ -f  $_  and print "$_ " } <$ENV{HOME}/MAIL/*>

Some people consider using map on empty context is bad form. I don't. They would propose instead:   for(<$ENV{HOME}/MAIL/*>) {  -f  $_  and print "$_ " }
or
  -f  $_  and print "$_ " for  <$ENV{HOME}/MAIL/*>

TMTOWTDI

-- stefp