use strict;
my $str = '<TERMEK>
<TERMEK_KOD>01008213</TERMEK_KOD>
<FO_KEP>https://www.temponabytok.sk/Files/obrazky/01008213.jpg</FO_KEP>
<OSSZESZERELESI_KEP>https://www.temponabytok.sk/Files/dokumenty/Vratko_montazny_navod.jpg</OSSZESZERELESI_KEP>
<OBRAZKY>https://www.temponabytok.sk/Files/2341000/2341428/245320712.jpg</OBRAZKY>
</TERMEK>';
my $regex = qr/(<FO_KEP>[^<]*<\/FO_KEP>)\r?\n(<OSSZESZERELESI_KEP>[^<]*<\/OSSZESZERELESI_KEP>)/p;
my $subst = '\\2\\n\\1';
my $result = $str =~ s/$regex/$subst/rg;
print "The result of the substitution is' $result\n";
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for Perl, please visit: http://perldoc.perl.org/perlre.html