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
[download]

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Update: Quotation bug fixed. (In the original version of my solution the RHS of the concat/assignment in the eval was the single-quoted '$_=\$$_ ', resulting in $combined ending up with the value 'a=$a c=$c' instead of 'a=1 c=1').

Also, to clarify what the eval is doing, at each iteration, only the $_ get interpolated; the remaining $'s are quoted verbatim. Hence, in the first iteration, the argument to eval is the string

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

All the a's in this string come from interpolating $_, whereas the $'s come from the \$'s in the original double-quoted expression.

The fuss with double quotes and backslashes is a way to selectively interpolate certain values (i.e. $_'s); without all the backslashing, perl would try to interpolate $$, for example.

Perhaps this is a clearer alternative:

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

Of course, in the last snippet, $template can alternatively be set to

'$combined .= "%s=$%s " if $%s'
[download]

or

'$%s and $combined .= "%s=$%s "'
[download]

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