use strict;
use warnings;

my %defs=(map {$_ => '-'}'a'..'p'); # generate simple example pairs
my $test1='a1 $a $q $c 2b';         # use undefined hash. => ERROR
my $test2='a1 $a $b $c 2b';         # only defined hashes => no ERROR

print '<'.convert($test1).">\n";
print '<'.convert($test2).">\n";

sub convert
{ $_=shift;
  s/\$([\w_\d]+)/unless(defined($defs{$1}))  
                 { print "ERROR: <\$$1> is not defined in ";
                   return($_);            # ok or bad style ??
                 } 
                 $defs{$1}
                /egx;
  return($_);
}
##</code><code>##

ERROR: <$q> is not defined in <a1 $a $q $c 2b>
<a1 - - - 2b>