use strict;
use warnings;
my %scalars;

# This is the long and wordy version of the line that follows this while loop.
while ( my $line = <> ) {
    my @scalars_on_this_line = ( $line =~ /(\$\w+)/g );
    foreach my $scalar ( @scalars_on_this_line ) {
        $scalars{ $scalar } = 1;
    }
}

# This is the faster (because of the hash slice )and smaller (undef values instead of lots of '1' values) method. I'd write it this way.
@scalars{ /(\$\w+)/g } = () while <>;

$\ = $, = "\n";
print sort keys %scalars;