my %allowed_text_code = (
   '$\\alpha$' => 'α',
   '$\\beta$'  => 'β',
   '$\\gamma$' => 'γ',
   '$\\delta$' => 'δ',
   '$\\theta$' => 'θ',
   '$\\pi$'    => 'π',
   '\\degrees' => '°'
);

my $text = join '', keys %allowed_text_code;

foreach my $tex_key (keys %allowed_text_code) {
   $text =~ s/\Q$tex_key\E/$allowed_text_code{$tex_key}/g;
}

open(my $fh, '>', 'temp.html') or die;
print $fh ("$text\n");