This sort of thing?

while ( my $line1 = <$fh> )
{
    my $line2 = <$fh>;

    # etc
}
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Except that

while ( my $line1 = <$fh> )
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is just the same as

while ( defined(my $line1 = <$fh>) )
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so perhaps I would do:

while ( my $line1 = <$fh> )
{
    defined(my $line2 = <$fh>) or last;

    # etc
}
[download]