use strict;
use warnings;

my $combined = '';
my ( $a, $b, $c ) = ( 1, 0, 1 );

eval qq(if ( \$$_ ) { \$combined .= "$_=\$$_ " }) for qw( a b c );

print "$combined\n";
__END__
a=1 c=1

##</code><code>##

'if ( $a ) { $combined .= "a=$a " }'

##</code><code>##

my $template = 'if ( $%s ) { $combined .= "%s=$%s " }';
eval sprintf $template, $_, $_, $_ for qw( a b c );

##</code><code>##

'$combined .= "%s=$%s " if $%s'

##</code><code>##

'$%s and $combined .= "%s=$%s "'