This will do what you want - I'm using <DATA> for test purposes. use <> for yours.

#!perl -w
use strict;

while (<DATA>) {
    chomp;
    print "current line=$_\n";
    if (/check next/) {
        my $pos = tell();
        my $testline = <DATA>;
        chomp $testline;
        print "'next line' = $testline\n";
        #do something...
        seek DATA, $pos, 0;
    }
    print "current line=$_\n";
}

__DATA__
line 1
line 2
line 3 check next
line 4 OK?
line 5
[download]

Anyone know of an abbottoire going cheap?