perl -0pi -e "s/(<tag1>foo</tag1>ns*<tag2>)[^n]*(</tag2>)/1${varWithLeadingNumber}2/" file.xml
perl -0pi -e "s/(<tag1>foo</tag1>ns*<tag2>)[^n]*(</tag2>)/g{1}${varWithLeadingNumber}g{2}/" file.xml

perl ... -e'... s/.../...$1$varWithLeadingNumber.../ ...'

perl ... -e"... s/.../...${1}${varWithLeadingNumber}.../ ..."

perl ... -pe'
   BEGIN { $val = shift; }
   ... s/.../...$1$val.../ ...
' "${varWithLeadingNumber}"

export varWithLeadingNumber
perl ... -pe's/.../...$1$ENV{varWithLeadingNumber}.../'

varWithLeadingNumber=varWithLeadingNumber
    perl ... -pe's/.../...$1$ENV{varWithLeadingNumber}.../'

s/...1.../.../

s/...(?:1).../.../