use warnings;
use strict;

sub unpack_a { print "a(@_)\n"; 11 }
sub unpack_b { print "b(@_)\n"; 22 }

my $RECORD_SEQ_ID = 'a';
local $_ = 0;
my @lines = 42;

my $res = do { no strict 'refs'; &{"unpack_$RECORD_SEQ_ID"} ($lines[$_]) };

print $res;