use strict;
use warnings;

my $text=q'
\Start
We have $\alpha$-helical to a $\beta$-sheet proteins and stuff.
The $\beta$-sheet structures relate to $\pi$ by several \degrees.
';

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

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