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

    # etc
}

##</code><code>##

while ( my $line1 = <$fh> )

##</code><code>##

while ( defined(my $line1 = <$fh>) )

##</code><code>##

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

    # etc
}