Sounds like a job for redo.

#!/usr/bin/perl -w
use strict;

my $fields_per_record = 4;

while(<>) {
    chomp;
    my @records = split " ", $_;
    if(@records < $fields_per_record) {
        last if not defined (my $next = <>);
        $_ .= $next;
        redo;
    }
    print "@records\n";
}
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Makeshifts last the longest.