my $SOURCE_ADDR = param('source_addr') || '.*?';
my $DEST_ADDR   = param('dest_addr')   || '.*?';
my $PROTO       = param('proto')       || '\w+';
my $SOURCE_PORT = param('source_port') || '\d+';
my $DEST_PORT   = param('dest_port')   || '\d+';
...
my $entry =~ /\A
    \d+\s+
    (\d+)\s+
    ($SOURCE_ADDR)\s+
    ->\s+
    ($DEST_ADDR)\s+
    ($PROTO)\s+
    ($SOURCE_PORT)
    \s+>\s+
    ($DEST_PORT)
/x;