#!/usr/bin/perl
use strict;
use warnings;

sub sa { print "sub sa\n"; }
sub sb { print "sub sb\n"; }

my $name = 'a';

# using the symbol table
$main::{'s' . $name}->();

$name = 'b';

# using symbolic references:
no strict 'refs';
&{"s$name"}();
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Be aware that this might open some security holes depending on your application. You have been warned!