IF
... and this is just a WAG because your description seems to be about two different actions -- the first merely printing back the name of $file1 (a variable -- perhaps 'C:\Users\MeinerzM\Desktop\MPS_files\wotis\soc2b2014099d07.210000' -- which was apparently instantiated OUTside the code you posted)...
whereas the second seeks to modify the contents of $file1 (albeit with syntax that makes little sense to me) since there is no escaped "r" (whatever that would be) in the filename.

But, if that's incorrect, let us know by clarifying your question(s).

If it is correct, consider the difference: your first code will merely echo the value of the variable; not the contents of the file whose name is in your var, where the second code example apparently tries to modify the file's contents.

C:\> perl -E "my $var = 'rstuv';  $var =~ s/r/FOO/; say $var;"
FOOstuv  
    # replaced the "r" with "FOO"... and would have done the same if $
+var were a filename;
    # that is, it would have changed the filename in $var, not the con
+tent of the file.
    #  Now that I've said it multiple times, does this make sense to y
+ou?

C:\> perl -E "my $var = 'rstuv';  $var =~ tr/r/FOO/; say $var;"
Fstuv    
    # Grossly oversimplified, the tr///ran out of "r"s to change

C:\> perl -E "my $var = 'rstuv';  my $newvar = $var =~ tr/r/FOO/; say 
+$newvar;"
1   
    # count of the replacement actions
[download]

And if you're still interested is some kind of action with a tr/// please read perldoc -f tr to see how to use it correctly.