my $input = "(55.950254, -3.187606)";
( my $output = $input ) =~ s{([-\d.]+),  # capture the first number to
+ $1
                             \s+         # at least one space
                             ([-\d.]+)   # capture the second to $2
                            }
                            {$2 $1}x;     # reverse $1 & $2

print $output;
[download]

See perlre and perlretut for help with regular expressions, and Regexp Quote-Like Operators in perlop for s///.

update: added references to documentation