if (-e "some_file") {
   # do your thing
}
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if (open COULDBE, "filename"){
    #process here
    close COULDBE;
}
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If it exists, I need to open it.

Just try and open it. The world won't come to an end if you try to open a file that doesn't exist, just trap the error.

  if( open( IN, '<', $somefile ) ) {
    # it's  there, do something with it
    close(IN);
  }
  else {
    warn "wasn't able to open $somefile (reason=$!)\n";
  }
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You might also want to consider using IO::File objects, which lend themselves to being passed to subroutines. Of course you can also pass typeglobs around, e.g.

    process(*IN);
    sub process {
      *FH = shift;
      while( <FH> ) {
          ....
      }
    }
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I do it all the time, but it is considered antiquated in some circles, as it's a throwback to the Perl 4 era.

You might also want to consider using IO::File objects ... you can also pass typeglobs

Or you can use lexical, autovivified filehandles. These are the variety I prefer:

    open my $in, "<", $filename or die "open $filename: $!";
    process($in);
    close $in;
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