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 content of the file.
    #  Now that I've said it multiple times, does this make sense to you?

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