I always figured, if you're going to do it, do it right:

#!/usr/bin/perl

use strict;

my $pattern = shift @ARGV;

my ($src, $dest) = 
    ($pattern =~ m/^s(.)([^\1]*)\1([^\1]*)\1$/)
    or die "1st arg not s/// construct (or construct too complicated)"
+;

my %to_rename;

for my $f (@ARGV) {
    (! -e $f) && die "$f does not exist; $!";
    my $n = $f;
    $n =~ s/$src/$dest/;
    ( -e $n or defined $to_rename{$n})
        && die "$f would be renamed to $n; clobber detected";
    $to_rename{$n} = $f;
}

while (my ($n, $f) = each %to_rename) {
    rename $f, $n or warn "Cannot rename $f to $n; $!";
}
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OK, I can see bad style there, but it works. I wrote it some time ago...

I called the script ~mv, and it is called as ~mv s/\.old$/.new/ *.old