use strict;
my $str = ' --color-primary: #0017a8;
--color-primary--rgb: #{hexToRGB(color-primary)};
--variablename: blabla blablabla;
--color-secondary: #0631ff;
--color-secondary--rgb: #{hexToRGB(color-secondary)};
--color-tertiary: #00ffe0;
--color-tertiary--rgb: #{hexToRGB(color-tertiary)};
--color-green-dark: #257426;
--color-green-dark--rgb: #{hexToRGB(color-green-dark)};
--font-family: TTCommons, Helvetica, Arial, sans-serif;
--transition: 0.3s ease;
--box-shadow: 0 0 12px 0 rgba(0, 0, 0, 0.3);';
my $regex = qr/(--([A-z\-]*?):(?<!--rgb:)).*?;/mp;
my $subst = '$1 #{$$$2};';
my $result = $str =~ s/$regex/$subst/rg;
print "The result of the substitution is' $result\n";
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for Perl, please visit: http://perldoc.perl.org/perlre.html