$ perl -le'
my $string = "MKTVEQTSPTMTSEKARWIYQKMVEIRMFEDRVHDIFSKGEIPGFVHLYAGEEAIA
+VGLCAHLDHNDYITSTHRGHGHCIAKGCELDGMMAEIYGKSTGLCKGKGGSMHIADLDRGMLGANGIVG
+GGFTLAAGAALTAKFKQTGGVAVCFFGDGANNQGTFHEGINLAAIWDLPVVFVAENNGYGEATPFHYAS
+ACEQITDRAKGYNIPGVKVDGKDVVAVYEVAREAVERARRGEGPTLIECITYRNYGHFEGDAQTYKTGR
+EKEEHTEERDAITLFEKYALSNNLLTEEAIQTVRHEVEQSVDRAVDFANASDYPQPEELLTDVYVSYDH
+EGRNG";
my $count;
$string =~ s/(?<=\G.{10})/ ++$count % 5 ? " " : "\n" /eg;
print $string;
'
MKTVEQTSPT MTSEKARWIY QKMVEIRMFE DRVHDIFSKG EIPGFVHLYA
GEEAIAVGLC AHLDHNDYIT STHRGHGHCI AKGCELDGMM AEIYGKSTGL
CKGKGGSMHI ADLDRGMLGA NGIVGGGFTL AAGAALTAKF KQTGGVAVCF
FGDGANNQGT FHEGINLAAI WDLPVVFVAE NNGYGEATPF HYASACEQIT
DRAKGYNIPG VKVDGKDVVA VYEVAREAVE RARRGEGPTL IECITYRNYG
HFEGDAQTYK TGREKEEHTE ERDAITLFEK YALSNNLLTE EAIQTVRHEV
EQSVDRAVDF ANASDYPQPE ELLTDVYVSY DHEGRNG
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