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use Algorithm::Diff qw( LCS_length );

...
    my ($lsa, $lsb) = @_;
    return LCS_length($lsa, $lsb) == @$lsa;
}

sub in_list {
    # Returns true if lsa is a proper subsequence of lsb
...
        ++( $lsa->[$ia] eq $lsb->[$ib] ? $ia : $ib );
    }
}