# GwenDragon
$ perl -E '$s = "<VT>BLA</VT>"; $s =~ s|(<VT>[^<]+)|${1}string2|; say $s'
<VT>BLAstring2</VT>

# MarkusH
$ perl -E '$s = "<VT>BLA</VT>"; $s =~ s/(<VT>)(.+)/${1}string2${2}/i; say $s'
<VT>string2BLA</VT>