You should have posted at least the full line of code, but I think I found what you're referring to at: floor.pl

    my $floor_number;
    for $floor_number (1..3) {
    $c->bind("floor${floor_number}", '<1>' =>    # line 64
        [\&floor_display, $floor_number, \%floor_labels, \%floor_items
+,
        \%cinfo, \$active_floor, $c_entry],
    );
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The first time through the loop, floor${floor_number} will become the literal string floor1 because of interpolation. The curlies are used to separate a variable from the strings around it without the need for whitespace. See Scalar value constructors.

For $$active_floor, see perlref