my $x = <<'EOX';
A {
  B 123
  C xyz
}
EOX
$x =~ s/A\s*\{.+?\}//s;
print "x='$x'\n"; # empty

##</code><code>##

perl -pe 's/A\s*\{.+?\}//s' <<'EOX'
A {
  B 123
  C xyz
}
EOX
# prints exactly what the input is

##</code><code>##

x='s/A\s*\{.+?\}//s'
echo $x
# s/A\s*\{.+?\}//s

##</code><code>##

cat > infile<<'EOX'
A {
  B 123
  C xyz
}
EOX
perl -pe 's/A\s*\{.+?\}//s' infile
# or perl -i -pe 's/A\s*\{.+?\}//s' infile