Oh, I see said the blind man to his deaf wife as he picked up his hammer and saw.

I suspect that something very simple like this would work:

use strict;
use warnings;

my $pattern ='ABC';

while (<>)
{
    last if /$pattern/;
}

# processing of rest of file starts here...

print "printing all lines after $pattern detected!\n";

while (<>)
{
    print;
}
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