In order to capture matches within a regular expression you'll have to give $id list context: 
my ( $id ) = $line =~ m/ID No: (\d+)/;

[download]

Should solve your problem

Unless I state otherwise, all my code runs with strict and warnings

In reply to Re: Removing duplicate lines based on a match by FunkyMonk
in thread Removing duplicate lines based on a match by carrerag

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